Data transmission systems are typically characterized by lines which extend from a transmitting station to receiving terminals. The lines may be in the form of a loop in which the transmission is always in the same direction, called a simplex loop, or they may be in a more complex loop or not in a loop at all, such as terminals which are connected over telephone lines. Whatever the form of connection between terminals it is desirable to have the capability of transmitting data messages at more than one speed; high speeds for purposes of throughput and lower speeds to provide the means to drop down in an attempt to eliminate faulty transmissions where for noise or some other reason received data does not correspond to the transmitted data. It is also desirable to have a plurality of speed choices in systems which do not normally operate over telephone lines but which can use telephone lines as a back up capability when the primary controller is not in operation. Since systems which are independent of telephone lines can be made to operate at speeds in excess of those available on telephone lines, it is necessary to have the capability of operating at different speeds in order to make use of such a back up system.
It is, therefore, the general object of this invention to provide the capability of changing speeds at each receiving terminal through automatic means to avoid the necessity of manually changing speeds whenever the data transmission rate changes.